Study Of Zn Loss Mechanisms And Inhibition Methods For Zn-based Catalysts For Acetylene Acetylation

Ethylene acetate is an important chemical raw material,which is widely used and the demand is increasing year by year.At present,there are two main process methods for industrial production of ethylene acetate,namely calcium carbide acetylene method and petroleum ethylene method.Due to the abundant coal resources in northwestern China,the use of calcium carbide acetylene method to synthesize ethylene acetate can significantly improve energy efficiency and comprehensive economic benefits.However,there are still some problems with the calcium carbide acetylene process.The catalysts currently used in industry,such as zinc acetate/activated carbon catalysts,are prone to deactivation,low productivity and high by-products,which still need further improvement and refinement.One of the most prominent problems is the short catalyst lifetime,and the loss of active component Zn in the catalyst is an important reason affecting the catalyst lifetime,but there are still few studies on the mechanism of catalyst active component loss.Therefore,this thesis is divided into the following three parts,starting from the study of the mechanism of active component Zn loss and then exploring the methods to inhibit the loss based on the mechanism.（1）The relationship between Zn loss and reaction temperature and air velocity was first investigated,and the rate of Zn loss was found to be directly proportional to the rate of ethylene acetate production.Under the same conditions,nitrogen,acetylene,acetic acid,vinyl acetate,acetylene and acetic acid were introduced to carry out the reaction,and the Zn content before the reaction was compared to compare the loss of Zn under different reaction atmospheres.It was found that the loss of Zn occurred only when vinyl acetate,acetylene and acetic acid were introduced,and it was also shown that the loss of Zn occurred only in the presence of vinyl acetate.Finally,the products of the reaction between Zn acetate and ethylene acetate alone were collected and the approximate structure of the substance was determined by using NMR,GC-MS,and FTIR characterization.The molecular structure of the substance,which has a molecule of acetate attached to one end of Zn and a molecule of ethylene acetate attached to the other end,is named here as zinc acetate-ethylene acetate.（2）Vinyl acetate forms a complex with Zn,which leads to the loss of Zn.Chelating agents can form more stable complexes with metal ions,and also have strong adsorption with activated carbon（AC）,which can inhibit the loss of Zn with vinyl acetate to a certain extent,and the preparation method is simple.Seven kinds of chelating agents with different ability to cooperate with Zn were screened.Controlling the pH of the solution below the zero charge point(pH_zpc)value of AC can largely enhance the adsorption ability of AC on Zn-chelating agent groups.Further studies were carried out for the introduction of ethylenediaminetetraacetic acid（EDTA）,and its optimum addition ratio was determined to be 2%,and the acetic acid conversion could reach 65%,too much EDTA would lead to a sharp decrease of Zn content on the catalyst.However,the appropriate ratio can enhance the dispersion of Zn while the reduction of Zn content is not significant.the introduction of EDTA will also enhance the adsorption capacity of acetic acid on the catalyst.After 180 h of reaction,the conversion of acetic acid remained above 60%and the loss of Zn was 7.17%,while the activity of Zn/AC decreased to 40%after 180 h and the loss of Zn was as high as57.42%.（3）Citric acid（CA）is rich in oxygen-containing functional groups,which can theoretically improve the activity of the catalyst to some extent.The conversion of acetic acid can be maintained above 70%at any CA mass fraction between 10%and 20%,and the inhibition of Zn loss is more effective.The catalytic activity can be further improved by calcination,up to 80%,but the calcination decomposes part of the CA,thus reducing the effect of Zn loss inhibition.After 200 h of activity test,the conversion of acetic acid was maintained at about 80%and the loss of Zn was only 7.25%（all catalysts tested in this thesis had a selectivity of above 99%for vinyl acetate）.